Ants use one parasite to fend off another

If you’re a fungus-growing ant patiently tending your garden, the last thing you want is to have your nest invaded by parasitic invader ants who make themselves at home by consuming your young. Unless those uninvited guests end up protecting you from other predator ants that are much worse. That’s the symbiotic relationship some host ants have reached with their parasites.

Sericomyrmex amabilis
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Let me introduce you to three kinds of ants. Sericomyrmex amabilis (the host ants, top) are farmers. Megalomyrmex symmetochus (guests, middle) are parasites that invade the gardeners’ nests. Once there, they feast on both the host brood and on their fungus garden, but slowly enough so that the colony isn't obliterated. Megalomyrmex also trap the host queens within the colony by clipping their wings, ensuring a steady supply of ant baby fodder. In this way, the Megalomyrmex set themselves up as permanent guests. Finally, there are Gnamptogenys hartmani (raiders, bottom). These marauding predators completely take over any fungus gardens they find, driving out or killing all the host ants.

Megalomyrmex symmetochus

Photographer: John T. Longino

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Rachelle Adams and her colleagues from the University of Copenhangen have found that while the host ants are ill equipped to drive away the raiders, their parasitic guests have more potent weaponry. Nests containing even a moderate number of these guest ants were completely protected from the raiding ants. Often raiding colonies will avoid nests with odors indicating the presence of the guest parasites.

Gnamptogenys hartmani

Photographer: Ryan Perry
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Needless to say, this puts the poor host ants in a difficult position. They have to accept the constant culling of their young and their resources or face the likely extinction of the entire colony. Clearly, the former is the lesser of two evils. Of course, no one is asking them what they think, least of all the researchers, who staged caged death matches between different numbers and types of ants.

Adams RM, Liberti J, Illum AA, Jones TH, Nash DR, & Boomsma JJ (2013). Chemically armed mercenary ants protect fungus-farming societies. Proceedings of the National Academy of Sciences of the United States of America PMID: 24019482.